1. Field of the Invention
The present invention relates generally to telephone equipment and particularly to data access arrangements (DAAs).
2. Background Art
Telephone systems were originally designed for voice communication. As new information technologies have emerged, methods have been devised to transmit other types of information over telephone lines. These methods have required the connection of other equipment besides the voice telephone set to the telephone line. Examples of such equipment include computer modems, facsimile ("fax") machines, answering machines, voice mail systems, phone patches, automatic number identification or "Caller ID" systems, and advanced telephone systems. The signals present within the above equipment often have different electrical characteristics than the signals that may be transmitted on a telephone line. Thus, an interface is required to connect such equipment to a telephone line and to translate between the equipment signals and the telephone line signals.
Traditionally, especially for computer modems, a DAA is used to connect equipment to a telephone line. A DAA is an electronic circuit that typically provides coupling, isolation, impedance matching, hybrid and sometimes amplification, filtering and control functions. Isolation refers to the separation of electrical signals present on the customer premises equipment (CPE) side of the DAA from those present on the telephone line or central office (CO) side of the DAA. Isolation is important to avoid damage to the equipment from voltages present on the telephone line, such as the central office battery voltage, inductive switching pulses, foreign electromotive force (EMF) and transients caused by electrical storms. Isolation is also required under Part 68 of the Federal Communications Commission (FCC) Rules to prevent damage to the public switched telephone network (PSTN) by equipment connected to it. However, the isolation should be provided in a manner that allows the desired signals to be communicated between the CPE side and the CO side of the DAA. Impedance matching refers to the adaptation of the electrical characteristics of the equipment to the intrinsic electrical characteristics of the telephone line. Impedance matching is important because it maximizes the efficiency of the transmission of signals along the telephone line. A hybrid refers to a device for splitting a bidirectional communication path into two unidirectional communication paths. A line carrying both the transmit side and the receive side of a conversation may be split into a transmit path and a separate receive path. A hybrid is important because it is often desirable to have a signal containing only the received signal without any interference from the transmitted signal. Amplification refers to increasing the amplitude of a transmitted or received signal. Amplification is important for matching the audio levels of the equipment and the telephone line and for compensating for line loss. Filtering refers to removing unwanted signals, especially unwanted high frequency signals, from the signals that pass through the DAA. Filtering is important to eliminate frequencies that might cause interference. Control refers to the selection of the status of the DAA and includes such parameters as on-hook/off-hook, gain and loopback.
One characteristic of a hybrid circuit is transhybrid loss. Transhybrid loss is the ratio of the amount of the transmit signal present in the bidirectional communication path to the amount of the transmitted signal present in the unidirectional receive signal path. Where the bidirectional path is a typical telephone line, transhybrid loss is the ratio of the amount of transmit signal present across the conductors of the telephone line to the amount of transmit signal present at the receive signal output. Ideally, none of the transmit signal should be present in the receive signal path, so a hybrid should have a high and stable transhybrid loss value.
DAAs have traditional been large and bulky. As computers become smaller and more portable, there is a need for smaller, more portable DAAs. Notebook and palmtop computers, for example, require a small, lightweight DAA for connection to a telephone line. A DAA contains certain parts that dissipate heat and other parts that are thermally sensitive. As the DAA size is reduced, the parts that dissipate heat must be placed closer to the thermally sensitive parts. The heat produced by the heat dissipating parts changes the performance of the thermally sensitive parts. Thus, a method is needed for reducing the thermal sensitivity of a DAA circuit for use in miniaturized applications.
The thermal sensitivity problem is exacerbated by the miniaturization of the individual components used to make a miniaturized DAA. A transformer is typically used to couple-the equipment side of a DAA to the telephone line side of a DAA. To provide a miniaturized DAA, the transformer must be miniaturized. To miniaturize the transformer, the diameter of the wires used to wind the transformer must be reduced and the number of windings must be increased. A reduction of the diameter and cross sectional area of the wires results in an increased winding resistance. Adding more windings increases the winding resistance further. Since the wires used to wind a transformer have a considerable temperature coefficient of resistance and the wires have a higher winding resistance, heating of the transformer results in a significant increase in winding resistance.
When power is first applied to a circuit with a thermally sensitive transformer, the winding has not been heated and has a relatively low resistance value. As power remains applied and components of the circuit dissipate heat, the transformer is heated and its resistance increases. Thus, the increased winding resistance is not necessarily a static elevation of resistance, but may be a dynamic variation of resistance over temperature.
The application of power to the circuit may involve the application of supply voltage to the circuit, the initiation of telephone line loop current flow, the transition of components from a relatively quiescent state to a relatively active state, or other changes that increase power dissipation of components in the circuit.
Certain types of circuits are particularly sensitive to changes in resistance of circuit components. For example, a high speed modem, such as a CCITT V.32 modem, performs an equalization and training sequence when a connection is first established. The training sequence sets modem parameters to the proper values for the characteristics of the modems and communication lines being used. However, a change in transformer winding resistance over time can lead to a change in transhybrid loss. If the transhybrid loss comes to have a different value than it had during the training sequence, the modem parameters must be adjusted to compensate for the change in transhybrid loss. The modem performs another training sequence to attempt to retrain the modem to the new transhybrid loss value. Retraining requires time and delays the transmission of data. Thus, a temperature compensation method is needed to prevent variation of transhybrid loss with temperature.
FIG. 1A illustrates a typical hybrid circuit using a traditional relatively large low resistance transformer. The first terminal of resistor RL is coupled to the TIP conductor. The second terminal of resistor RL is coupled to the first terminal of AC voltage source VR. The second terminal of AC voltage source VR is coupled to the RING conductor. The TIP conductor is coupled to the first terminal of the first winding of transformer T1. The RING conductor is coupled to the second terminal of the second winding of transformer T1.
Input +TX is coupled to the first terminal of resistor RT. The second terminal of resistor RT is coupled to node V1, which is coupled to the first terminal of the second winding of transformer T1 and to the first terminal of resistor R3. Input -TX is coupled to the second terminal of the second winding of transformer T1 and to the first terminal of resistor R2. The second terminal of resistor R2 is coupled to the second terminal of resistor R3, to the inverting input of amplifier ARX and to the first terminal of resistor R1. The second terminal of resistor R1 is coupled to the output of amplifier ARX and to output RX.
A differential transmit signal having a voltage equal to two times VTX is applied across inputs +TX and -TX. The signal passes through resistor RT and the second winding of transformer T1. Transformer T1 is a 1:1 matching and isolation transformer with input and output impedances typically in the 600-900 .OMEGA. range. Resistor RT has a value approximately equal to the impedance of the transmission line. Resistor RT is in series with the differential input +TX/-TX and provides a matching impedance of a nominal value of RL. With ideal components (including a transformer having zero winding resistance) and with RT equal to the transmission line impedance, the transmit signal has no effect on the voltage at node V1. However, under less than ideal transmission line conditions, the transmit signal across differential input +TX/-TX affects the voltage at node V1, but the echo canceller of a V.32 or other modem may be used to determine the amount of transmit signal present at node V1 and to provide for any signal processing needed to subtract the transmit signal present with the receive signal.
The voltage at node V1 is expressed as follows: ##EQU1## assuming R3&gt;&gt;RT+RL.
Amplifier ARX and resistors R1, R2 and R3 form a summing amplifier circuit. The amplifier circuit produces a signal at output RX that is proportional to the difference between the voltage present at the non-inverting input of amplifier ARX and the sum of the voltages of node V1 and input -TX. Under ideal conditions where RT=RL, even if a transmit signal is present across inputs +TX and -TX, it is canceled out by the hybrid circuit so that it does affect the voltage at output RX, which is referred to as VRX.
When a transmit signal is applied across inputs +TX and -TX and is coupled to the second winding of transformer T1, transformer T1 inductively couples the desired AC signals from the second winding to the first winding while isolating any DC voltages, DC offsets and/or common mode voltages of the windings. Thus, the desired AC components of the transmit signal are applied across the TIP and RING conductors. The TIP and RING conductors represent a differential transmission line, preferably a twisted pair telephone line.
Voltage source VR represents the AC signal source at the opposite end of the telephone line, which is typically located at the telephone company central office. Resistor RL represents the resistance of the TIP and RING conductors summed with the source impedance of voltage source VR. A receive signal is represented by the variation of the voltage of voltage source VR. The differential receive signal is applied across the TIP and RING conductors and appears across the first winding of transformer T1. Transformer T1 passes the desired AC components of the receive signal from its first winding to its second winding while isolating any DC voltages, DC offsets and/or common mode signals. The receive signal across the second winding of transformer T1 appears across the inputs to the summing amplifier circuit. Although the summing amplifier circuit is adjusted to provide a zero voltage output when no receive signal is present, the presence of a receive signal across the inputs to the summing amplifier causes the summing amplifier to change its output voltage in response to the receive signal. Output RX tracks the receive signal and is not affected by a transmit signal, even if one is present. Thus, the hybrid separates the receive signal from the transmit signal and provides a receive signal output independent of any transmit signal. The hybrid allows both the transmit and receive signals to be transmitted along the TIP and RING conductors without interference from each other.
The voltage VRX has the following value: ##EQU2##
G1 and G2 can be defined as follows: ##EQU3##
If G1 and G2 are substituted into equation (2) and equation (1) is substituted into equation (2), the following equation results: ##EQU4##
In equation (3), VTX can be eliminated if the following equation is satisfied: ##EQU5##
If VTX is eliminated, equation (3) can be rewritten as follows: ##EQU6##
FIG. 1B illustrates a hybrid circuit constructed with a transformer having a high winding resistance, which can result from long windings and/or windings of small cross-sectional area. FIG. 1B is substantially identical to FIG. 1A except FIG. 1B has resistor RW inserted between node V1 and the first terminal of the second winding of transformer T1. Node V1 is coupled to the first terminal of resistor RW. The second terminal of resistor RW is coupled to the first terminal of the second winding of transformer T1.
Some transformers, particularly those with long windings and/or those wound with wire of small cross-sectional area, have significant winding resistance, represented by resistor RW. In telephony, return loss measures how well the impedance of CPE, such as a hybrid circuit, matches the nominal telephone line termination impedance. To optimize return loss, resistor RT is selected to have a value that, when added to winding resistance RW, equals the characteristic line impedance (typically 600 ohms for telephone lines). However, since the value of resistor RT no longer matches the characteristic line impedance, some of the transmit signal is present at node V1 even if the hybrid circuit is coupled to an ideal transmission line. To compensate for the attenuated +TX signal present at node V1, amplifier ARX is configured as a summing amplifier to add an attenuated version of signal from the -TX input to the signal from node V1. The sum of the attenuated +TX and -TX signals is zero, thereby eliminating any components of the transmit signal from node V1. Thus, only the receive signal is amplified by amplifier ARX.
With the proper component selection and the absence of drift in component values, the circuit of FIG. 1B can provide optimum transhybrid loss for a particular telephone line impedance. If the impedance of the transmission line is different than expected or if the component values (including the transformer winding resistance) change, the transhybrid loss changes.
Resistor RW is shown in series with the ideal second winding of transformer T1, but actually represents the significant DC resistance of the second winding since the second winding is not an ideal winding. The DC resistance of the winding is dependent upon the resistivity of the material of which the winding is constructed, its length, cross-sectional area and temperature. The resistivity, length and cross-sectional area are generally fixed under normal operation. The temperature, however, can and usually does vary. Often, certain components of a hybrid circuit dissipate substantial amounts of heat. To maintain a telephone line in an off-hook state, DAAs typically use a holding circuit or so-called "holding coil." The holding circuit allows DC current to pass, indicating to the telephone company central office that the telephone line is off hook. The holding circuit does not pass AC current, thus allowing other DAA components to receive and transmit AC signals without interference. The DC holding current is dependent upon the length of a telephone line from the central office, the type of cable used for the telephone line and the type of central office equipment. Typical phone lines in the United States provide about 40 mA of DC current. However, DC currents ranging from about 20 mA to about 120 mA may be expected. At 120 mA, a holding circuit typically drops about 15 volts across it, thereby dissipating about 1.8 watts of power. In a small DAA package, such dissipation may increase the temperature of the DAA circuitry by about 15 C. It often takes 15-20 minutes for such a DAA to reach a state of thermal equilibrium. Although the dissipation usually occurs in the holding circuit components, the temperature of the transformer windings may also increase, especially if located in close proximity. As the temperature of the windings changes, the resistance of the windings also changes. The temperature coefficient of the transformer winding resistance RW is typically about 4000 ppm/C. The temperature coefficient of resistor RT is typically much less than 4000 ppm/C. Since the transhybrid loss is a function of these resistors and since the resistors have greatly different temperature coefficients, the transhybrid loss of the DAA changes significantly as it warms up. Higher holding current levels cause more temperature rise, which results in greater transhybrid loss drift.
Transhybrid loss refers to the amount of transmit signal present at the receive signal output RX. The function of the hybrid is to minimize the amount of the transmit signal present at the receive signal output. Thus, the transhybrid loss should be minimized. Transformer winding resistance is one factor that can contribute to increased transhybrid loss. Although it is possible to compensate for the winding resistance, variations in transhybrid loss over time can impair communications. As transformer windings warm up and their resistance changes, the resulting changes in transhybrid loss can cause a reduction in signal to noise ratio. In CCITT V.32 data modems, transhybrid loss drift may cause data errors and may cause the modem to initiate a retraining sequence to adjust the equalization parameters to values suitable for the changed levels of transhybrid loss. Retraining requires time and reduces the throughput and reliability of the modem connection.
The voltage V1 of FIG. 1B is given by the following equation: ##EQU7##
The voltage VRX of FIG. 1B is given by the following equation: ##EQU8##
To minimize transhybrid loss, the following equation must be satisfied: ##EQU9##
However, since RW, which is dependent upon temperature, cannot be cancelled from equation (8), equation (8) cannot be satisfied at more than a single temperature using the circuit of FIG. 1B. Thus, the circuit of FIG. 1B cannot maintain an ideal transhybrid loss over a broad temperature range.